This invention relates to emergency light control and battery charging systems and, more particularly, to such a system which provides extended battery life and high efficiency.
In commercial establishments, such as stores and office buildings, emergency lighting systems are often required under municipal building codes and safety ordinances. Such systems typically include a battery which is maintained in its charged state whenever conventional electrical AC power is available. When such power fails, emergency lights are turned on and powered from the battery until the conventional power is restored. When power is restored, the battery is recharged.
While such systems have been in fairly widespread use in the past, such systems suffer from a number of drawbacks. For example, such systems typically utilize a linear type of voltage regulation in the battery charging circuit. This results in a relatively large transformer and the requirement that the voltage regulator must have a heat sink.
Another drawback in such prior systems is due to the fact that the battery needs a slightly higher charging voltage when the ambient temperature is low and a slightly lower charging voltage when the ambient temperature is high. The reason for critical battery voltage control is to extend the life of the battery. In emergency lights, the battery must last from three to five years in the standby mode and constantly trickle charging a battery will cause it to fail in a few months. The energy from the trickle charger, after a battery is fully charged, goes into converting its internal chemicals into unusable compounds or electroplating its terminals into useless stubs. Another problem is that a battery can be discharged and still show full voltage. For example, a 6 volt battery will measure 6 volts even when discharged, so that to charge this battery more than 6 volts must be applied to it for current to flow. At 75.degree. F, the battery can be fully charged at 6.8 volts, whereas the battery will be destroyed at 7 volts.
Accordingly, it is an object of this invention to provide an improved emergency light control and battery charging system.
It is a further object of this invention to provide such a system where the voltage regulator is switched, rather than linear, to allow a lower wattage and smaller transformer to be utilized and to avoid the requirement that the voltage regulator must have a heat sink.
It is another object of this invention to adjust the charging voltage based on ambient temperature in order to achieve longer battery life.